diff-tree: convert diff_tree_sha1 to struct object_id

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/merge.c b/builtin/merge.c
index a4a098f..afaed6a 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -415,7 +415,7 @@ static void finish(struct commit *head_commit,
 			DIFF_FORMAT_SUMMARY | DIFF_FORMAT_DIFFSTAT;
 		opts.detect_rename = DIFF_DETECT_RENAME;
 		diff_setup_done(&opts);
-		diff_tree_sha1(head->hash, new_head->hash, "", &opts);
+		diff_tree_oid(head, new_head, "", &opts);
 		diffcore_std(&opts);
 		diff_flush(&opts);
 	}